Metallic nanocrystalline materials and metallic glasses exhibit relatively high hardness and strength characteristics for metal-based materials and because of this, they are considered to be potential candidates for structural applications. However, their limited fracture toughness and ductility associated with the rapid propagation of shear bands and/or cracks may be a concern for the commercial utilization of their superior strength. Typically, these materials may exhibit adequate ductility by testing in compression while tensile ductility of the same materials may be close to zero. At the same time, tensile ductility along with fracture toughness is understood to be a relatively important characteristic for structural applications where intrinsic ductility is needed to avoid catastrophic failure.
Nanocrystalline materials may be understood to be or include polycrystalline structures with a mean grain size below 100 nm. They have been the subject of widespread research since mid-1980s when it was asserted that metals and alloys, if made nanocrystalline, may exhibit a number of appealing mechanical characteristics of potential significance for structural applications. But despite relatively attractive properties (high hardness, yield stress and fracture strength), it is understood that they may show a disappointingly low tensile elongation and may tend to fail in a relatively brittle manner. In fact, empirical correlations between the work hardening exponent and the grain size for cold rolled and conventionally recystallized mild steels indicate a decrease in ductility for decreasing grain size. As the grain size is progressively decreased, the formation of dislocation pile-ups may become more difficult, limiting the capacity for strain hardening. That may lead to mechanical instability of materials under loading.
Attempts to improve the ductility of nanocrystalline materials while maintaining their relatively high strength by adjusting the microstructure have also been made. It has been proposed that an increased content of high-angle grain boundaries in nanocrystalline materials could be beneficial to an increase in ductility. In a search to improve ductility of nanocrystalline materials, extremely ductile base metals have been used. For example, nanocrystalline Cu with a bimodal grain size distribution (100 nm and 1.7 μm) has been fabricated based on the thermomechanical treatment of severe plastic deformation, which may exhibit a 65% total elongation to failure and may retain a relative high strength. Recently, a nanocrystalline Cu with nanometer sized twins embedded in submicrometer grained matrix by pulsed electrodepositon has been produced. The ductility and relatively high strength may be attributed to the interaction of glide dislocations with twin boundaries. In a recent approach, nanocrystalline second-phase particles of 4-10 nm were incorporated into the nanocrystalline Al matrix (about 100 nm). The nanocrystalline particles interacted with the slipping dislocation and enhanced the strain hardening rate which leads to the evident improvement of ductility. Using these approaches, enhanced tensile ductility has been achieved in a number of nanocrystalline materials such as 15% in pure Cu with mean grain size of 23 nm or 30% in pure Zn with mean grain size of 59 nm. It should be noted that fracture strength of these nanocrystalline materials does not exceed 1 GPa. For nanocrystalline materials with higher fracture strength (1 GPa) the achievement of adequate ductility (>1%) may still be a challenge.
Amorphous metallic alloys (metallic glasses) represent a relatively young class of materials, having been first reported around 1960 when classic rapid-quenched experiments were performed on Au—Si alloys. Since that time, there has been progress in exploring alloys compositions for glass formers, seeking elemental combinations with ever-lower critical cooling rates for the retention of an amorphous structure. Due to the absence of long-range order, metallic glasses may exhibit relatively unique properties, such as relatively high strength, high hardness, large elastic limit, good soft magnetic properties and high corrosion resistance. However, owing to strain softening and/or thermal softening, plastic deformation of metallic glasses may be highly localized into shear bands, which may result in a limited plastic strain (less than 2%) and may lead to catastrophic failure at room temperature.
Different approaches have been applied to enhance ductility of metallic glasses such as introducing free volume in amorphous structure or glassy phase separation which has enabled up to 25% in compression. However, tensile ductility for these materials has not been reported. Another approach is the development of metallic glass matrix composites. Crystalline precipitates may be introduced into a glass matrix by partial crystallization. Crystallization occurs by nucleation and growth mechanism and depending on glass composition and crystallization kinetics, nanometer-sized or micrometer-sized crystallities might be introduced.
This approach may also allow an increase in compressive ductility in Ti-based, Zr-based, Mg-based glasses and Cu—Hf—Ti—Nb system. Tensile ductility of these materials was demonstrated up to 13% tensile elongation in Ti—Zr-based metallic glasses with large dendrites (20-50 μm in size) embedded in the glassy matrix. The heterogeneous structure of these composites may act as an initiation site for the formation of shear bands and/or a barrier to the rapid propagation of shear bands, which leads to enhancement of global plasticity, but sometimes decreases the strength.
Another way to reduce grain size is through spinodal decomposition which may occur when a mixture of two or more materials separate into distinct regions with different material concentrations. This method differs from nucleation in that phase separation due to spinodal decomposition may occur throughout the material, and not just at nucleation sites. Spinodal decomposition was previously observed in AlNiCo magnets, 17-4PH stainless steel, Fe-25Cr-12Co-1Si alloy, and Fe-based austenitic alloy. Recent studies mentioned a Co enrichment in the amorphous residual matrix and Fe enrichment in the α′-FeCo crystalline phase. In addition, experimental evidence of grain refinement caused by the formation of clusters, which, in turn, resulted from the addition of >1% Cu has been presented. It has also been shown that Cu additions of more than 1.0% promoted the formation of clusters responsible for the grain refinement of the crystalline α′-FeCo phase, i.e., grain size around 10 nm. However, no property evaluation of the final structure was performed in these studies. With respect to AlNiCo magnets, while it is known from a number of sources that relatively high tensile strengths can be obtained from 28 to 380 MPa, the material response may be somewhat brittle and tensile elongation data is generally not listed.